The present invention relates to a method and to portable apparatus for detecting gas by selective absorption of electromagnetic radiation to detect the presence of a particular gas within a mixture of gases.
Gas leak detectors are characterized by three properties, namely: selectivity, i.e. the ability to detect a particular gas amongst a mixture of gases present in an atmosphere; sensitivity, i.e. the minimum quantity of gas that can be detected; and stability, i.e. insensitivity to variations in climatic conditions so as to ensure that performance remains constant regardless of climatic conditions.
Various types of detector are already known for detecting a particular gas, such as methane, in an atmosphere made up of a mixture of gases.
Thus, semiconductor detectors exist in which a combustible gas reacts on coming into contact with the semiconductor, thereby reversibly altering the electrical resistance of the semiconductor, and this resistance is very easy to measure. That type of sensor is of low cost and is used above all for detecting leaks in the home. It has medium performance and stability, but no selectivity.
Catalytic detectors can be used for detecting the presence, e.g. of methane, and they are fitted to detector appliances for detecting methane at concentrations in a measurement range of several hundred parts per million (ppm) to several percent, by volume. Although such detectors have acceptable stability, their selectivity is very poor.
Thermal conductivity detectors use the ability of a gas to evacuate heat. The presence of a gas such as methane gives rise to a variation in thermal conductivity, and this variation is measured. Such appliances are not selective and they are not adapted to measuring low concentrations of gas, e.g. less than 1% by volume of methane.
Flame ionization appliances make use of the fact that hydrocarbon flames conduct electricity. The presence of a hydrocarbon such as methane modifies the conductivity of a hydrogen flame between two electrodes. Sensitivity is good and response times are short. Thus, appliances of that type can be used to measure in the range 1 ppm to several hundred ppm with good stability. However selectivity is zero.
Known infrared optical detectors present medium performance in terms of sensitivity and selectivity.
Various types of non-dispersive infrared (NDIR) type gas detectors are known. Nevertheless, most gas analyzers using standard NDIR type techniques lose their effectiveness when the gases to be detected and measured present absorption bands that are non-specific and overlap in the infrared range.
FIGS. 3 and 3A are diagrams showing an example of an optical type gas analyzer as described in U.S. Pat. No. 4,914,719.
In such an embodiment, a source 10 of infrared radiation powered with alternating current (AC) produces a beam of infrared radiation which passes through a chamber 14 containing a sample of a gas mixture, and it impinges on a beam splitter 12. The beam splitter 12 directs a fraction of the incident radiation to a wheel 16 carrying filters, and the fraction of the infrared radiation which passes through the wheel 16 carrying filters 22, 24, and 26 is picked up by a photodetector 20.
A stepper motor 18 rotates the wheel 16 so as to position the various filters 22, 24, and 26 in turn between the beam splitter 12 and the detector 20.
The fraction of the infrared radiation that passes through the beam splitter 12 passes initially through an interference filter 30 and is then picked up by a photodetector 28.
FIG. 4 shows typical curves representing the transmission spectrum as a function of wavelength for three gases A, B, and C having absorption bands that overlap. It can be observed that a standard NDIR type gas detection technique using a bandpass filter centered on wavelength xcex and presenting a half-maximum bandwidth xcex94xcex, as shaded in FIG. 4 is incapable of distinguishing between the three gases A, B, and C insofar as all three gases A, B, and C present various absorption bands in this zone of the spectrum. Insofar as a standard technique makes it possible to perform a transmission measurement only, it is possible to obtain only one equation having three unknowns (three gas concentrations).
The apparatus shown in FIGS. 3 and 3A enables this problem to be remedied by placing three bandpass filters 22, 24, and 26 on the wheel 16, the filters having narrow bands centered on wavelengths xcex1, xcex2, and xcex3, with respective bandwidths xcex94xcex1, xcex94xcex2, and xcex94xcex3 thus enabling a set of three equations to be obtained. Under such circumstances, the filter 30 is itself connected in such a manner as to correspond to a reference beam centered on the wavelength xcex0 which is close to the characteristic absorption wavelength of the gases present in the chamber 14, but which does not overlap these characteristic absorption wavelengths.
The prior art apparatus of FIGS. 3 and 3A thus provides a set of four measurement signals that can be used to detect the concentration of three different gases. The apparatus can be adapted to detecting the concentrations of N different gases providing N filters 22, 24, and 26 are selected that are centered on different wavelengths.
Although such an apparatus as known from U.S. Pat. No. 4,917,719 enables a plurality of gases to be detected simultaneously, it is not adapted to detecting a particular gas simply and quickly using a portable appliance. The apparatus described above with reference to FIGS. 3 and 3A has moving parts, in particular the rotary disk 16, which increases the weight and the size of the apparatus and also the amount of energy it consumes, while also making the apparatus relatively fragile, particularly in the presence of vibration. Furthermore, such apparatus can be used only on condition that the composition of the gas mixture for analysis is known in advance, and it needs to be calibrated for each gas whose concentration is to be determined within the mixture of gases.
Other types of NDIR gas analyzer are known, that implement a gas filter correlation (GFC) technique. By way of example, such gas analyzers are described in the work entitled xe2x80x9cTechniques and mechanisms in gas sensingxe2x80x9d by P. T. Moseley, J. O. W. Norris, and D. E. Williams, published by Adam Hilger, Bristol, Philadelphia, and New York.
In that technique, the gas to be measured is used in high concentration as a filter for the infrared radiation passing through the chamber of the gas analyzer that is filled with the gas mixture to be analyzed. The basic components of a gas analyzer using the GFC technique and designed to measure ambient carbon monoxide CO are shown in FIG. 5.
The infrared radiation emitted by a source 40 is chopped and then passes through a gas filter which comprises in alternation a reference filter 42 containing a high concentration of a gas of the same kind as that which is to be detected in the mixture of gases (such as carbon monoxide), and a measurement gas filter 43 containing nitrogen in this example. The gas filters 42 and 43 pass in alternation in front of the source 40, given that they are placed on a support which is rotated by a motor 41 defining the chopping of the beam from the source 40. Once the beam of radiation has passed through the gas filter device 42, 43 it can pass through an additional bandpass filter 44 and then penetrates into a chamber 45 containing the mixture of gases and within which absorption occurs due to the gases in the mixture of gases. Once the infrared beam has passed through the chamber 45 it reaches a photodetector 47, after passing through a lens system 46 that performs focusing and that also contains an interference filter having a narrow passband. The detector 46 is associated with electronic processor circuits 48 and with a display device 49.
If the chamber 45 does not contain a gas that causes infrared radiation to be absorbed and if the radiation from the source xcex0 passes through the CO filter 42, then the spectrum of the light intensity received by the detector 47 as a function of wavelength has the form shown in FIG. 6A. The dashed line envelope of the curve is due to the presence of the narrow band filter, while the notches are due to individual spectrum lines in the absorption spectrum of CO. The shaded zone corresponds to the total energy contained in the beam. The CO filter has eliminated all of the radiation that could be absorbed by CO such that the CO present in the chamber 45 cannot further reduce the energy in the beam. Nevertheless, other gases present in the chamber 45 and having absorption spectra that overlap that of CO can absorb energy from the infrared beam. The CO filter 42 serves to produce a reference beam.
In contrast, when the infrared beam passes through the nitrogen filter 43, no energy is absorbed by the nitrogen, and in the absence of any absorbent gas in the chamber 45, the spectrum of the signal reaching the detector 47 is in the form shown by dashed lines in FIG. 6B, which form is due to the presence of a bandpass filter. The measurement beam that has passed through the nitrogen filter 43 and the reference beam that has passed through the carbon monoxide filter 42 can be brought into equilibrium by means of a neutral density filter, for example. When the chamber 45 does not contain any absorbent gas, the balanced energies of the beams that have passed through the filters 42 and 43 lead to a zero difference signal being detected.
In the event of the chamber 45 containing a mixture of gases that include carbon monoxide, the carbon monoxide cannot absorb radiation in the reference beam but can absorb radiation in the measurement beam, thereby leading to spectra shown respectively in FIGS. 6C and 6D. The energy difference between the reference beam and the measurement beam leaving the chamber 45 leads to an output signal being issued that represents the concentration of CO in the chamber 45.
Absorption by an interfering gas present in the measurement cell 45 causes the apparatus of FIG. 5 to give a positive output signal for the regions of the spectrum of the interfering gas where there is overlap with the absorption lines of CO, and its gives a negative signal for regions of the spectrum of the interfering gas where there is no overlap with the absorption lines of CO.
Thus, in the presence of an interfering gas, there exists a weak residual output signal whose sign and amplitude depend on the sample and on the overlap of the absorption spectra of the gas to be measured (CO) and of the interfering gas.
It can be seen that a measurement technique of the kind set out in the above-specified work provides more effective elimination of the influence of interfering gases than do the apparatuses described in U.S. Pat. No. 4,914,719, and, in addition, does not require calibration for each specific gas.
Nevertheless, the apparatuses described in the above-cited work also make use of moving parts such as the rotary wheel supporting the gas cells that form filters, and such apparatuses cannot be implemented in a form that is simultaneously compact, robust, and not greedy for energy.
The present invention seeks to remedy the above-specified drawbacks and to enable gas detectors to be made that present simultaneously: excellent sensitivity to a predetermined gas; good selectivity relative to said predetermined gas; good robustness against shocks; low weight; and small size, enabling it to be implemented in portable form and at low cost, while not requiring any special maintenance.
The invention also seeks to provide a gas detector that has good stability against temperature variations.
These objects are achieved by a portable apparatus for detecting gas by selective absorption of electromagnetic radiation to detect the presence of a particular gas in a gas mixture, the apparatus comprising:
a) first emitter means for emitting a measurement infrared electromagnetic radiation beam in a wavelength band containing a wavelength at which the gas to be detected presents an absorption characteristic;
b) second emitter means for emitting a reference infrared electromagnetic radiation beam, said first and second emitter means being activated in alternation;
c) a measurement cell containing the gas mixture to be analyzed, which measurement cell has an inlet section and an outlet section and receives at least a fraction of the measurement beam through its inlet section;
d) a filter cell containing a sample of the gas to be detected and having an inlet section and an outlet section, through which sections at least a fraction of the reference beam passes in succession;
e) first detector means for detecting electromagnetic radiation beams;
f) second detector means for detecting electromagnetic radiation beams;
g) a beam splitter disposed in such a manner as to split firstly the measurement beam and secondly the reference beam so as to transmit a first fraction of each of the measurement and reference beams to said first detector means, and a second fraction of each of the measurement and reference beams to the second detector means; and
h) processing and acquisition means for synchronously acquiring and processing the four signals (US1, US2, UR1,UR2) delivered by the first and second detector means in succession when the first and second emitter means are respectively activated in order to determine the absolute concentration of the gas to be detected on the basis of the ratio R=(US1xc3x97UR2)÷(US2xc3x97UR1) between said four signals where US1 and US2 respectively represent the signals delivered by the first and second detector means when the first emitter means is activated, and where UR1 and UR2 respectively represent the signals delivered by the first and second detector means when the second emitter means is activated.
In a first embodiment, the filter cell containing a sample of gas to be detected has inlet and outlet sections through which the entire reference beam passes in succession; the beam splitter is placed in such a manner as to transmit the first fraction of each of the measurement and reference beams to said first detector means through the inlet and outlet sections of the measurement cell; and the second detector means is placed in such a manner as to receive the second fraction of the measurement and reference beams directly, the beam splitter being arranged in such a manner as to receive the reference beam after it has passed through the outlet section of the filter cell.
In another embodiment, the measurement cell containing the mixture of gases to be analyzed comprises an inlet section and an outlet section through which the entire measurement beam passes in succession; the beam splitter is placed in such a manner as to transmit the first fraction of each of the measurement and reference beams directly to said first detector means; and the second detector means is placed in such a manner as to receive the second fractions of the measurement and reference beams via the inlet and outlet sections of the filter cell, the beam splitter being arranged in such a manner as to receive the measurement beam after it has passed through the outlet section of the measurement cell.
The apparatus of the invention further comprises means for measuring the temperature of the medium in which the component elements of the apparatus are placed, and the signals delivered by said temperature measuring means are applied to said signal processing means for determining the absolute concentration of the gas to be detected.
The first and second emitter means can comprise light emitting diodes or laser diodes or solid lasers.
Modulator and filter means are associated with the emitter means. The modulation means are synchronized in such a manner that the first and second emitter means emit radiation in turns. The inlet and outlet sections of the measurement cell can be situated on the same side of the measurement cell or they can be on opposite sides.
The first and second emitter means can have emission spectra that are similar or different.
The invention also provides a method of detecting a gas by selective absorption of electromagnetic radiation to detect the presence of a particular gas within a mixture of gases, the method comprising the following steps:
a) emitting a measurement infrared electromagnetic radiation beam in a wavelength band containing a wavelength at which the gas to be detected presents an absorption characteristic;
b) emitting a reference infrared electromagnetic radiation beam;
c) modulating the emission of the measurement and reference beams in synchronized manner such that pulses of the measurement beam alternate in time with pulses of the reference beam;
d) causing at least a fraction of the measurement beam to pass through a measurement cell containing the mixture of gases;
e) causing at least a fraction of the reference beam to pass through a filter cell containing a sample of the gas to be detected;
f) separating the reference beam and the measurement beam into first and second fractions;
g) measuring the energies of the first fractions;
h) measuring the energies of the second fractions of the measurement and reference beams; and
i) determining the absolute concentration of the gas to be detected by using the four signals (US1, US2, UR1, UR2) a representing the energy measured in the first and second fractions of the measurement and reference beams as delivered in succession when the pulses of the measurement and reference beams are emitted respectively, using the ratio R=(US1xc3x97UR2)/(US2xc3x97UR1) between said four signals in which US1 and US2 respectively represent the energy measurement signals of the first and second fractions of the measurement and reference beams when the pulses of the measurement beam are emitted, and UR1 and UR2 respectively represent the energy measurement signals of the first and second fractions of the measurement and reference beams when the pulses of the reference beam are emitted.
The temperature of the medium in which the measurement and reference beams propagate is measured and the value determined for the absolute concentration of the gas to be detected is corrected as a function of the measured temperature.
In a particular implementation, the entire reference beam is passed through the filter cell containing a sample of the gas to be detected; said splitting is performed on the measurement beam and on the reference beam after it has passed through the filter cell; and the energies of the first fractions of the measurement and reference beams are measured after said first fractions have passed through the measurement cell containing the mixture of gases.
In another particular implementation the entire measurement beam is passed through the measurement cell containing the mixture of gases; said beam splitting is performed on the reference beam and on the measurement beam after it has passed through the measurement cell; and the energies of the second fractions of the measurement and reference beams are measured after these second fractions have passed through the filter cell containing a sample of the gas to be detected.
The method and the portable apparatus of the invention are particularly applicable to detecting methane, and sensitivity can be of the order of 1 ppm, for example, while selectivity can be very high when using infrared electromagnetic radiation.
Apparatuses of the invention can thus advantageously be used as a replacement for flame ionization appliances which suffer from the particular drawback of having no selectivity and of requiring gas cylinders containing a mixture of nitrogen and hydrogen in order to operate, whereas apparatuses of the invention require practically no maintenance. Furthermore, the absence of moving components or of components that require high energy consumption, such as electric motors, enables the invention to be used for making portable detectors that are compact, robust, and self-contained.